A former firefighter recounted in heartbreaking detail today how he owes his life to a hero of “Black Sunday,” one of the bleakest days in FDNY history.

The occasion was the unveiling of a plaque at Rescue 3 in The Bronx honoring Lt. Joseph DiBernardo, 40, who survived for six pain-filled years after falling 40 feet from a burning Bronx building and breaking every bone below his waist. Two of his fellow firefighters died that fateful day, as did a third in a separate blaze in Brooklyn.

DiBernado’s heroism — he lowered a fellow firefighter from the fourth-floor burning hell before leaving himself — has been recounted numerous times over the years.

And it was again today in moving tributes by Mayor Bloomberg, Fire Commissioner Sal Cassano and other fire officials.

But only one of the speakers, ex-firefighter Jeff Cool — the man who probably wouldn’t be alive today but for DiBernardo’s act of selfishness — was there.

“I know that you (can) always look to the plaque on the wall to remember Joey’s act of bravery and supreme sacrifice,” said Cool, choking back tears. “Me, all I have to do is look in the mirror and see myself.”

Cool recalled how DiBernardo, who was single, urged him to go first because “you have a wife and kids.”

“I think back to that day often and how Joey placed my life before his,” Cool said. “The courage he exemplified — we speak of brotherhood often in the fire service and I witnessed it first hand.”

Cool also suffered severe injuries, but recovered. DiBernardo didn’t.

His father, retired Asst. Deputy Chief Joseph DiBernardo Sr., recalled how his son became enraptured with firefighting as a young boy and had accumulated so much knowledge about the FDNY by age 11 that he could pass the lieutenant’s exam.

Both father and son were among those who rushed to Ground Zero after 9/11.

“The thing I think about every day,” said DiBernardo Sr.,”the two of us on the pile, father and son doing what they love to do. How many fathers get to do that?”

At times sad, at times funny, he ended up speaking for more than 20 minutes about the son he had raised to become a life-saver.

“Jeff stands here before you today because that little boy who loved toy fire engines and grew up to drive them, would rather go with his father to work rather than playing ball, the little boy who played with cardboard fire buildings and ended up leaping out of one for his life…grew up to be a hero himself,” DiBernardo Sr. told the audience, which spanned generations of firefighters.